The present invention relates to microstrip antennas, and more particularly, to a microstrip antenna having an improved bandwidth capability.
In general, antenna design is dependent upon the desired functional capabilities such as transmitting/receiving linearly polarized, right-hand circularly polarized, left-hand circularly polarized, etc., RF signals with the required gain, bandwidth, etc. Frequently it is desirable for an antenna to remain light in weight, simple in construction and unobtrusive where the antenna is to be mounted upon another structure such as a high velocity aircraft, or missile which cannot tolerate excessive deviations from aerodynamic shapes.
A microstrip antenna formed by etching a single side of a unitary metallically clad dielectric sheet (PC board) utilizing conventional photoresist etching techniques is generally recognized as a design which can be really adapted to meet the aforementioned desirable characteristics. Such an antenna may be only 1/32 inch to 1/8 inch thick and may be manufactured at minimum cost with maximum reproducibility and operating reliability. The cost to the customer is minimized since single antenna elements and/or arrays of such elements together with appropriate RF feedlines, phase shifting circuits, multiplexers, and/or impedance matching networks may all be manufactured as integrally formed electrical circuits utilizing low cost photoresist etching processes.
Numerous microstrip antennas have been developed which are advantageously suited to transmit/receive RF radiation having predetermined polarizations such as linear polarization and left-hand or right-hand circular polarization. Many of these have been designed for use in an overall array comprising a plurality of such individual microstrip antenna elements which are phased relative to one another to provide high gain fan beam or pencil beam radiation patterns when disposed in flat or even curved configurations. The necessary relative phase shifts for such microstrip antenna arrays can be economically achieved with phase shifting circuitry that is integrally formed in the PC boards by photoresist etching techniques. The fan or pencil beam of radiation may also be controllably steered by controlling switchable diodes or other controlled elements mounted directly on the microstrip structure in combination with appropriate integrally formed phase shifting circuits, etc.
Linearly polarized RF electromagnetic radiation may be produced by simply feeding the RF signal energy to one point along one side of a square shaped or rectangularly shaped microstrip radiator. Circularly polarized RF electromagnetic radiation may be produced by driving adjacent sides of a square microstrip radiator with signals having relative phasing of 90.degree. to produce the required conjugate phasing of the radiated fields. Either left-hand or right-hand circularly polarized signals may be produced. Other microstrip radiator elements may be driven to produce circularly polarized RF electromagnetic radiation. For example, a circular shaped microstrip radiator driven at points separated by 90.degree. relative phase angles will also produce the desired circularly polarized radiation.
Whether a microstrip antenna is adapted for use with linearly polarized radiation or circularly polarized radiation, the necessary RF feedlines, phase shifters, and/or impedance matching networks are usually integrally formed by conventional photolithographic techniques along with the radiating elements. Generally the dimensions of the RF feedlines are designed according to conventional impedance matching techniques to match the antenna impedance to the impedance of the anticipated coaxial cable or other RF conduits connected to the RF feedlines on the microstrip antenna structure.
While a wide variety of microstrip antennas have been developed, generally their operating frequency bandwidth is relatively small. Typically it is one or two percent when calculated by taking the difference between the upper and lower frequencies between which attenuation is not more than 3.0 decibels greater than its average attenuation through its passband and dividing that difference by the center frequency of the passband. It would be desirable to provide an improved broadband microstrip antenna having an operating frequency bandwidth substantially greater than the normally attained one or two percent values.
U.S. Pat. No. Re 29,911 discloses in FIG. 4 a circularly polarized microstrip antenna including a circular radiating element connected to a quadrature feedline impedance matching network. U.S. Pat. No. 3,665,480 discloses in FIGS. 1 and 2 a microstrip antenna including two overlapping dielectric layers, a front conducting plate on the upper surface of one of the layers, a back conducting plate on the bottom surface of the other one of the layers, an annular slot, and conductive feedlines extending between the conductive layers. U.S. Pat. No. 3,739,386 discloses various antennas including concentric slotted radiating elements. U.S. Pat. Nos. 2,996,610; 3,475,755; and 3,810,183 disclose other forms of non-microstrip antennas including ring-shaped radiating elements. Representative of the microstrip antenna field are U.S. Pat. Nos. 3,803,623; 3,971,125; 4,012,741; 4,053,895; 4,054,874; 4,079,268; 4,125,837; 4,125,838; 4,125,839; 4,160,976; 4,163,236; and 4,170,012. Also of general interest in this field, although generally of less pertinence than the patents already listed, are U.S. Pat. Nos. 3,016,536; 3,478,362; 3,680,136; 3,707,711; 3,823,404; 4,131,892; and 4,131,893.